1. Field of the Invention
The present invention relates to a method and apparatus for transmitting/receiving a signal in a communications system, and in particular to a method for transmitting/receiving a signal by an LDPC (Low Density Parity Check) code.
2. Description of the Related Art
The next generation communications system has been developed as a packet service communications system, which is a high capacity transmission system, to transmit burst packet data to a plurality of mobile stations. As the channel code effectively used for the next generation communications system, the LDPC code, along with the turbo code, has been positively considered for its good performance gain in high speed data transmission and improved reliability attained through effective correction of noise-related errors. Examples of the next generation communications system are the IEEE (Institute of Electrical and Electronic Engineers) 802.16e and 802.11n communications systems.
Referring to FIG. 1, there is described the structure of the signal transmission apparatus of a conventional communications system using the LDPC code. The signal transmission apparatus consists of an encoder 111, a modulator 113, and a transmitter 115. The information vector s to be transmitted is encoded by the encoder 111, according to a prescribed encoding method, into a codeword vector c, i.e., LDPC codeword, delivered to the modulator 113. In this case, the prescribed encoding method is the LDPC encoding. The modulator 113 modulates the codeword vector c, according to a prescribed modulation method into the modulated vector m delivered to the transmitter 115. The transmitter 115 processes the modulated vector m to generate the transmission signal transmitted to the signal receiving apparatus.
Referring to FIG. 2, there is described the structure of the signal receiving apparatus of a conventional communications system using the LDPC code. The signal receiving apparatus consists of a receiver 211, a demodulator 213, and a decoder 215. The receiver 211 processes the transmission signal received through the antenna to generate the receiving vector r delivered to the demodulator 213, which demodulates it, according to the demodulation method matching the modulation method of the modulator 113, into the demodulated vector x delivered to the decoder 215. The decoder 215 decodes the demodulated vector x according to the decoding method matching the encoding method of the encoder 111 so as to finally generate the recovered information vector ŝ.
As described above, the LDPC code has advantages, as in the turbo code, that the performance gain is superior at high speed data transmission and the reliability of the data transmission is improved by effective correction of the noise-related errors. The cycle of the LDPC code on the factor graph means the loop consisting of the edge connecting the variable node and the check node on the factor graph of the LDPC code, and the size of the cycle is defined as the number of edges. Accordingly, the size of the cycle increases with the number of the edges connecting the variable node and the check node. Thus, the performance of the LDPC code is improved as the cycle size of the LDPC code is increased on the factor graph.
Conversely, as the number of small-sized cycles of the LDPC is increased on the factor graph, the LDPC code suffers performance degradation due to error floor phenomena, impairing the error correction capability. Hence, if there are many small-sized cycles of the LDPC code on the factor graph, the information starting from an arbitrary node belonging to a small-sized cycle returns to itself after a small number of repetition, and thus, the increase of the repetition number results in the increase of the information returns, so that the information update is hindered so as to impair the error correction capability.
Moreover, the LDPC code is more complicated in encoding than the turbo code, making it difficult to encode in real time. In order to reduce the encoding complexity of the LDPC code, a repeated accumulation (RA) has been proposed, but the RA code also suffers limitations in reducing the encoding complexity of the LDPC code.
Therefore, when transmitting/receiving signals using LDPC code in a communications system, a solution is demanded to reduce the encoding complexity of the LDPC code together with enhanced error correction capability.